Alkali metal pyrophosphate solutions and methods of forming coatings therewith



United States Patent ALKALI METAL PYROPHOSPHATE SOLUTIONS AND METHODS OF FORMING COATINGS THEREWITH Werner Rausch and Willy Werner, Frankfurt am Main,

Germany, assignors to Parker Rust Proof Company,

' Detroit, Mich., a corporation of Michigan No Drawing. Application January 15, 1958 Serial No. 708,993 1 The invention relates to improved alkali metal pyrophosphate coating solutions and to a method of forming corrosion-resistant phosphate coatings therewith in short periods of time. Y

It has heretofore been suggested that aqueous acidi pyrophosphate solutions could be used as substitutes for aqueous acidic orthophosphate solutions. The conditions for operating such pyrophosphate solutionsaud the oxidizing agents indicated to be useable therewith were substantially similar to those conventionally used by the art with aqueous acidic orthophosphate solutions. Such pyrophosphate solutions are now known to be inferior to orthophosphate solutions when operated under the conventional conditions for orthophosphate solutions. For example, no coating was formed as the result of contacting a steel surface with an aqueous solution containing 12 grams/liter of disodium pyrophosphate, 0.6 gram/liter sodium nitrite and 30 grams/liter sodium chlorate when the solution was maintained at a temperature of 158 F., and the contact time wa-s'between 20 and 30 seconds.

This invention is based on the discovery that alkali metal pyrophosphate solutions are capable of producing highly useful, corrosion-resistant coatings when those solutions are operated under specific conditions of acidity and in the presence of certain selected oxidizing agents, and when operated under such conditions form coating in relatively short periods of time which are commercially useable in continuous high quantity production operations. It has also been found, in accordance with this invention, that the coatings resulting from the use of such aqueous acidic pyrophosphate solutions are greatly improved in their corrosion-resistant ability by the formation on the surface of such coating of a CrO layer when the thickness of that CrO layer is restricted within certain defined f Broadly stated the method of forming phosphate coatings in short periods of time, that is periods of about 5 seconds to about 1 minute, is performed by contacting a ferrous surface with an aqueous acidic alkali metal pyrophosphate solution containing an alkali metal nitrite and having a pH in the range of about 2.7 to about 3.7. The corrosion-resistance of such coatings is significantly improved by contacting such coated metal surface with a dilute aqueous hexavalent chromium-containing solution of sufiicient concentration and under application conditions which produces on that coated surface a hexavalent compound containing overlayer. The CrO content of the overlayer depends on what chemical is used in the hexavalent chromium-containing solution. 'When using chromic acid the overlayer may contain CrO in the range of 4-30 mgJsquare meter, when using dichromates the CrO content in the overlayer is at least 7 rug/sq. meter, preferably at least 10 mg./ square meter.

1 The aqueous acidic pyrophosphate solutions of this invention may satisfactorily contain about 0.6 gram/ liter ICE to about 8 grams/liter pyro=P O and may satisfactorily be formed from the alkali metal pyrophosphate salts such for example as disodium pyrophosphate, trisodium pyrophosphate or the corresponding potassium pyrophosphates. It is preferred to employ the sodium salts and particularly the disodium pyrophosphate salt. The solutions of this invention preferably contain about 1 to about 6 grams/liter pyro-P O content. While concentrations greater than about 8 grams/liter can be employed, no additional benefit is obtained. The solutions of this in vention may contain a proportion of orthophosphate in admixture with the pyrophosphate, but the solution should, in all cases be predominately a pyrophosphate solution. It is possible to obtain satisfactory coatings, when the ratio of ym-P 0 content to ortho-P O con tent is 1:1, but is preferred that the ortho-P O content be limited to amounts represented by the ratio of .1 part ortho-P O to 2.5 parts ym-P 0 In order to form coatings in periods of time of, for example, between about 5 to 60 seconds, it is necessary to adjust the acidity of the solution to a pH value with the range of about 2.7 to about 3.7 and to concurrently introduce in the solution an amount of an alkali metal nitrite in the range of about 0.1 to about 1 gram/liter. Aqueous acidic solutions containing these quantities of pyrophosphate and alkali metal nitrite operate to form uniform blue-gray phosphate coatings on ferrous sur-' faces when operated at temperatures of about 85 F. to about 160 F. for about 5 to about 60 seconds and preferably for about 10 to about 30 seconds.

The above indicated acidity range is important to the attainment of the improved coatings of this invention. Attempts to form coatings on ferrous surfaces with aqueous sodium pyrophosphate solutions having a pH representing the pH developed by merely dissolving disodium pyrophosphates in water, that is between 4.2. and 4.8, containing sodium nitrite and operated in the above stated range of temperature, were unsuccessful. It was found to be necessary to increase the acidity of such solutions to a pH within the range of about 2.7 to about 3.7 in order to obtain coatings in times of less than a minute. When the pH value was 2.5, a yellowish coating was formed of inferior characteristics to the bluish coatings obtained when the pH was within therange of 2.7 to 3.7. When the pHof the solution was 4.2 and 5.1, contact of clean ferrous surfaces under the above con- 7 ditions failed to produce any visible coating. The coatings were most uniform and of the desirable blue-gray appearance when the pH did not numerically exceed about 3.7. Any source of hydrogen ions which does not inhibit the coating-forming ability of the pyrophosphate solution may be used to achieve the above specified range the rate of metal attack in orthophosphate solutions were not satisfactory for use in the aqueous acidic pyrophos-. phate solutions of this invention. For example, attempts to use potassium permanganate, nitro quanidine, and

. sodium hypochlorite in proportions ranging from .l to

10 grams/liter and sodium chlorate in quantities of v 1 to 10 grams/liter in solutions containing 3.2 grams/liter of pyro-P O at a pH of 3.0, a temperature of 113 I by contacting ferrous surfaces for 20 seconds, failed ,to

L produce satisfactory coatings. In comparison,

Patented June 23, 19 59 wise comparable solutions containing sodium nitrite in a concentration between 0.1 gram/liter and l gram/liter produced uniform iridescent blue coatings in 20 seconds. The nitrite is preferably added to the operating solutions in the form of a dilute aqueous solution and may be added periodically, but preferably is continuously trickled into the solution.

The solutions of this invention can be satisfactorily applied by dipping the articles to be coated in the solution. or by spraying the solution on the surface of the articles. Some benefit has been observed from brushing the surface of the articles during immersion when the dipping method of application is employed. As articles are removed from the dipping bath the excess coating solution is preferably removed from the surface by using a squeeze-off device, such as a pair of opposed rolls.

In making up the operating baths of this invention, it is preferred to employ an admixture of the alkali metal pyrophosphate and an alkali metal hydrogen sulfate in proportions such that the resulting acidity is numerically lower in pH than the desired operating minimum of 2.7. Such a solution is brought within the desired operating range by the addition of tetra-sodium pyrophosphate or a conventional alkalizing material such as sodium carbonate. As the solution is continuously used the pH value has a tendency to rise and periodically it is necessary to add acid to adjust the operating pH within the above given range. For any particular operating conditions it is possible to originally formulate the relative proportions of pyrophosphate and bisulfate in a solid make-up material so that the same material may be used for continuously replenishing the depleting components as the solution is used. For most operations, a combined make-up and replenishing material can be formulated to include between 30% to 70% disodium pyrophosphate and 70% to 30% sodium hydrogen sulfate with the particular quantities being easily established by starting with a 50-50 admixture and modifying after short term use to best reflect the particular operating conditions in use. As above indicated, the presence of ortho-P O content in the operating solution is undesirable and as the solution normally tends to form ortho-P O during use, it is desirable to observe the solution periodically for the formation of a loose, powdery overlayer n the adherent phosphate coating and when such a layer is detected the solution normally contains an excess quantity of ortho-P O One method of avoiding the build-up of excessive quantities of ortho-P O is to employ small containers for the solution and to process large quantities of material through the solutions so that the ortho- P O which is removed from the solution on the surface of the coated work is sufficient to maintain the desired balance between the pyro-P O content and the ortho-P O content. If this method is not feasible the solution can be re-balanced by adding pyro-P O The presence of excess quantities of ortho-P O in the operating solution is illustrated in the specific example designated Example I.

EXAMPLE I A plurality of solutions were formulated to contain a total P 0 content of 5 grams/ liter, 0.5 gram/ liter sodium nitrite and each was adjusted to have a pH value of 3 to 3.3 by the addition of sulfuric acid thereto. in the solutions the ratio of pyro-P O to ortho-P O was varied by altering the relative proportions of sodium dihydrogen phosphate and disodium pyrophosphate while keeping constant the P 0 content to a total of 5 grams/liter. The solutions were heated to a temperature of about 120 F. and cold rolled steel sheets were immersed for seconds therein, withdrawn, rinsed and inspected. The nature of the coating obtained as the result of using the various ratios of pyro-P O to ortho-P O are set forth in the table designated Table I and this table additionally illustrates the efiect of brushing the surface while imniersed.

4 Table I The formation of a layer PYIO-PgOsZOlthCtin dipping with Simul- P O In Dipping taneous brushing of the metal sheet during the treatment.

9 Very good, powder free Very good, powder free. do Do. Good, powder free Good, powder free. Medium, powdery Do. do Medium, powdery. O 25 Medium, very powdery Do.

M the corrosion resistance of the coated article relative to an uncoated surface the corrosion resistance has been found to be greatly enhanced by the contact of the coated surface with an aqueous hexavalent chromium containing solution. Moreover, it has been found that while the strength of the hexavalent chromium-containing solution may vary over a relatively wide range, the best corrosion results are obtained when the quantity of CrO contained in the resultant coating lies within a narrow range of coating weights. The hexavalent chromiumcontaining solution can be one formed from chromic acid or from a heavy metal dichromate, such as zinc dichromate, calcium dichromate, cadmium dichromate, aluminum dichromate or admixtures thereof. It has been found, however, that the minimum effective weight of hexavalent chromium in the coating which is obtained from solutions formulated from CrO is somewhat different from the minimum effective weight of CrO in the hexavalent chromium-containing coating when that coating is derived from a solution formulated from a heavy metal dichromate. The CrO content of the overlayer depends on what chemical is used in the hexavalent chromium-containing solution. When using chromic acid the overlayer may contain CrO in the range of 4-30 mg./square meter, when using dichromates the CrO content in the overlayer is at least 7 mg./square meter, preferably at least 10 mg./ square meter.

It will be apparent to those skilled in the art that the weight of the hexavalent chromium-containing compound on the metal surface will be dependent both upon the concentration of the solution used and upon the mechanical operation of the apparatus used to squeeze off the excess hexavalent chromium-containing solution as the treated surface emerges from the rinsing bath. Example II illustrates typically satisfactory concentrates of hexavalent chromium in the rinse solutions and also shows the difference between dichromates and chromic acid solutions, and compares the corrosion resistance obtained with that of a non-rinsed surface.

EXAMPLE II A phosphate solution was prepared to contain 6.2'

grams/liter sodium hydrogen sulfate, 3.8 grams/liter disodium pyrophosphate, 0.6 gram/liter sodium nitrite and was adjusted to a pH value of 2.8 with sodium carbonate. A plurality of low-carbon steel sheets were preliminarily cleaned in an aqueous hydrochloric acid,

solution, rinsed in water and contacted with the pyrophosphate solution for 5 seconds at a temperature of 113 F.122 F., withdrawn, rinsed with water and then rinsed with a hexavalent chromium-containing solution for 3 seconds at a temperature of F.- F.,, squeezed to remove the excess hexavalent chromiumcontaining solution and then dried. The panels were then observed for the appearance of rust, in air, after the varying periods of time indicated in Example II. The

degree of rusting was evaluated on a numerical basis ex tending from 1 to 15 with the numbers from 1 to 5 signifying increasing light rust, from 6 to 10 indicating increasing misting, and from 11 to 15 indicating increasing;

thorough rusting.

. -1min,

Table II connosron mum. t gammm, METAL ssnnnrs HANGlNG nnnnmr Treatment 1 OrO; on the metal sheet surface,

nag/sq. it.

Mam-

Oorroslon Rating-aiter days NO Pyrfiphosphate-nltrlte,

grams of disodium pyrophosphate Na,H,P,0, in

a liter of water.- -A-'sufiicient' "ainount' of sodiumnitrite (NaNO was then-addedto each of the bathsolutions so that the first bath solution had a NaNO- content of 0.1 gram/liter, the second of 0.2 gram/liter, the third of 0.4 gram/liter, the fourth of 0.6 gram/liter and the fifth of 1.0 gram/liter. The five solutions were adjusted to a pH value of 3.0 by the addition of sulfuric acid (H SO and then brought to a temperature of 115 F. Steel panels were immersed in each bath for a period of seconds and on each panel there was formed a uniformly distributed iridescent blue phosphate coating.

The very best operating conditions which illustrate the preferred form of this invention are represented by the use of the phosphate solution of this example containing 0.6 gram/liter of sodium nitrite and rinsing the phosphate coating formed from the use of that solution in a zinc dichromate aqueous acidic solution containing 1.4 grams/liter of zinc and 4.3 grams/liter CrO by immersing the coated article in such solution at a temperature of 140 F. for 3 to 5 seconds, withdrawing and rinsing the same with 'water.

What is claimed is:

1. An aqueous acidic solution for forming a corrosionresistant coating on a metallic surface which comprises an alkali metal nitrite in an amount in the range of about 0.1 gram/liter to about 1.0 gram/liter, expressed as sodium nitrite, an alkali metal pyro-P 0 content in the range of about 0.6 gram/liter to about 8 gram/liter, said solution having a pH in the range of about 2.7 to about 3.7.

2. An aqueous acidic solution for forming a corrosionresistant coating on a metallic surface which comprises an alkali metal nitrite in an amount in the range of about 0.1 gram/liter to about 1.0 gram/liter, expressed as sodium nitrite, a P 0 content in the range of about 0.6 gram/liter to about 8 gram/liter, said solution having a pH in the range of about 2.7 to about 3.7, said P 0 content being derived from alkali metal pyrophosphate and orthophosphate and said pyrophosphate being the predominant contributor to said P 0 content.

3. A solution in accordance with claim 2 wherein the ratio of pyro-P O to ortho-P 0 is at least 2.5: 1.

4. An aqueous acidic solution for forming a phosphate coating on a ferrous surface which consists essentially of water, an alkali metal pyrophosphate and an alkali metal nitrite, the ym-P 0 content of said solution being in the range of about 0.6 to about 8.0 grams/liter, said alkali metal nitrite being present in an amount of about 0.1 gram/liter to about 1.0 gram/liter, expressed as sodium nitrite, said solution having a pH in the range of about 2.7 to about 3.7.

5. An aqueous acidic solution for forming a phosphate coating on a ferrous surface which consists essentially of'water, an alkali metal pyrophosphate and an alkali metal nitrite, the ym-P 0 content of said solution being in the range of about 1 gram/liter to about 6' grams/liter, said alkali metal nitrite being present in an amoun't'ofabout 0L1 gram/liter to about 1.0 gram/ liter, expressed as sodium nitrite, said solution having a pHi'n the range of about 2.7 to about 3.7."

6. A solution in accordance with claim 4 wherein said pyro-P O is derived from disodium pyrophosphate and said alkali metal nitrite is sodium nitrite.

7. A method of forming a corrosion-resistant coatin-g upon a metallic surface which comprises the steps of applying to said surface an aqueous acidic solution comprising an alkali metal nitrite in an amount in the range of about 0.1 gram/liter to about 1.0 gram/liter, expressed as sodium nitrite, a P 0 content in the range of about 0.6 gram/liter to about 8 grams/liter, said solution having a pH in the range of about 2.7 to about 3.7, said P 0 content being derived from alkali metal pyrophosphate and orthophosphate and said pyrophosphate being the predominant contributor to said P 0 content, and maintaining said solution in contact with said surface until an adherent coating is formed thereon.

8. A method of forming a corrosion-resistant coating on a ferrous surface which comprises the steps of applying to said surface an aqueous acidic solution comprising an alkali metal nitrite in an amount in the range of about 0.1 gram/liter to about 1.0 gram/liter, expressed as sodium nitrite, a P 0 content in the range of about 0.6 gram/liter to about 8 grams/liter, said solution having a pH in the range of about 2.7 to about 3.7, said P 0 content being derived from alkali metal pyrophosphate and orthophosphate and said pyrophos phate being the predominant contributor to said P 0 content, and maintaining said solution in contact with said surface until an adherent coating is formed on said surface.

9. A method for forming a phosphate coating on a ferrous surface which comprises the steps of applying to said surface an aqueous acidic solution consisting essentially of water, an alkali metal pyrophosphate and an alkali metal nitrite, the pyro-P O content of said solution being in the range of about 0.6 to about 8.0 grams/liter, said alkali metal nitrite being present in an amount of about 0.1 gram/liter to about 1.0 gram/liter, expressed as sodium nitrite, said solution having a pH in the range of about 2.7 to about 3.7, and maintaining said solution in contact with said surface until an adherent coating is formed on said surface.

10. A method of forming a corrosion-resistant coating on a metallic surface which comprises the steps of applying to said surface an aqueous acidic solution comprising an alkali metal nitrite in an amount in the range of about 0.1 gram/liter to about 1.0 gram/liter, expressed as sodium nitrite, an alkali metal pyro-P O content in the range of about 0.6 gram/liter to about 8 grams/liter, said solution having a pH in the range of about 2.7 to about 3.7, maintaining said solution in contact with said surface until an adherent coating is formed on said surface, applying to the coated surface a dilute aqueous hexavalent chromium-containing solution, removing the excess hexavalent chromium-containing solution from said surface and drying said surface so as to deposit in said coating a quantity of hexavalent chromium-containing compound, expressed as CrO in the range of about 0.37 ing/sq. ft. to about 4.0 ing/sq. ft.

11. A method of forming a corrosion-resistant coating on a metallic surface which comprises the steps of applying to said surface an aqueous acidic solution comprising an alkali metal nitrite in an amount in the range of about 0.1 gram/liter to about 1.0 gram/liter, expressed as sodium nitrite, a P content in the range of about 0.6 gram/liter to about 8 grams/liter, said solution having a pH in the range of about 2.7 to about 3.7, said P 0 content being derived from alkali metal pyrophosphate and orthophosphate and said pyrophosphate being the predominant contributor to said P 0 content, maintaining said solution in contact with said surface until an adherent coating is formed on said surface, applying to the coated surface a dilute aqueous hexavalent chromium-containing solution, removing the excess hexavalent chromium-containing solution from said surface and drying said surface so as to deposit E5. in said coating a quantity of hexavalent chromium-containing compound, expressed as Cro in the range, of about 0.37 mg./sq. ft; to about 4.0 mg/sq. ft.

12. A method of forming a corrosion-resistant coating on a metallic surface which comprises the steps of applying to said surface an aqueous acidic solution comprising water, an alkali metal pyrophosphate and an alkali metal nitrite, the pyro-P O content of said solution being in the range of about 0.6 to about 8.0 grams/ liter, said alkali metal nitrite beingrpresent in an amount of about 0.1 gram/liter to about 1.0 gram/liter, expressed as sodium nitrite, said solution having a pH in the range of about 2.7 to about 3.7, maintaining said solution in contact with said surface until an adherent coating is formed on said surface, applying to the coated surface a dilute aqueous hexavalent chromium-containing solution, removing the excess hexavalent chromiumcontaining solution from said surface and drying said surface so as to deposit in said coating 9. quantity of hexavalent chromium-containingcompound, expressed as CrO in the range of about 0.37 mg./sq. ft. to about 4.0 mg./sq. ft.

References (Jilted in the file ofthis-patent UNITED STATES PATENTS 2,067,007 Darsey Jan. 5, 1937 2,499,261 Rosenbloom Feb. 28, 1950 2,715,059 Miller Aug. 9, 1955 FOREIGN PATENTS 517,049 Great Britain Jan. 18, 1940 726,935 Great Britain Mar. 23, 1955 

1. AN AQUEOUS ACIDIC SOLUTION FOR FORMING A CORROSIONRESISTANT COATING AN A METALLIC SURFACE WHICH COMPRISES AN ALKLI METAL NITRITE IN AN AMOUNT IN THE RANGE OF ABOUT 0.1 GRAM-LITER TO ABOUT 1.0 GRAM/LITER, EXPRESSED AS SODIUM NITRITE, AN ALKALI METAL PYRO-P2O5 CONTENT IN THE RANGE OF ABOUT 0.6 GRAM/LITER TO ABOUT 8 GRAM/LITER, SAID SOLUTION HAVING A PH IN THE RANGE OF ABOUT 2.7 TO ABOUT 3.7 